System and method for flue gas purification for thermal power units

ABSTRACT

A system and method for purification of combustion gases cools the combustion gases to an optimal oxidation temperature, oxidizes the combustion gases, injects water or uses natural moisture content to form acid aerosols, neutralizes the acid aerosols with an alkaline solution, and then filters the acid aerosol, alkaline solution and water from the combustion gases. The purified gas stream is then exhausted from the system. Oxidation can be provided in stages to obtain optimal oxidation of different combustion gas components with different optimal oxidation temperature ranges. When used in a thermal power plant, the cooling of exhaust gases can be accomplished by regenerative preheating of combustion air, boiler feedwater or combustion fuel, and the clean exhaust gas can be cooled below the conventional exhaust stack temperature range, thus enhancing power plant efficiency.

FIELD OF THE INVENTION

The inventions described herein relate to the field of pollutioncontrols and thermal power plant operation.

BACKGROUND OF THE INVENTION

Thermal power plants are designed to produce electrical power with steamdriven turbines wherein the steam is produced by burning fuel to heatwater in a boiler. The fuels used in thermal power plants includes coal,oil, and natural gas and combinations thereof, and in some casesbagasse, wood chips, or even rubber tires. Principal products ofcombustion include excess oxygen, nitrogen, carbon dioxide, and watervapor. Other by-products of combustion include particulate and ash,sulfur dioxide and sulfur trioxide (for sulfur bearing fuels), oxides ofnitrogen, and carbon monoxide. These by-products are harmful to theenvironment and are considered as pollutants.

The pollution control systems described herein are directed at removingsome or all of these pollution components from combustion gases ofthermal power plants. The systems are applicable to thermal power units,pulp and paper incinerators, or any exhaust gas from a fuel burningplant. However, some embodiments are ideally suited for use in theoperation of thermal electrical generation plants because a significantincrease in boiler efficiency may be obtained.

FIG. 1 schematically illustrates various features of a typical thermalpower unit including heat recovery equipment from the exit flue gas.Combustion of the fuel in the firebox 1 produces heat to create steam inthe boiler 2 from a closed cycle of purified, recirculating watersupplied from the feedwater system 3, and boiled in the boiler 2. Thesteam is used to operate a turbine 4 and generator 5 to produceelectrical power. Preheat is usually added to the feed water byfeedwater heaters 6 using extraction steam from various stages of theturbine. The water is introduced into the boiler by high pressure feedpumps where it flows through heat exchange tubes 7 contained on theinside surfaces of the boiler's combustion chamber or firebox 1, inwhich continuous combustion takes place at high temperature.

Fuel is injected into the firebox using a fuel injection systemcomprising a high pressure fuel pump and fuel line (not shown).Combustion air is injected into the firebox with a combustion airinjection system comprising high capacity blowers in the form of forceddraft fans 8 and/or induced draft fans 9 and regenerative air preheaters13. The resulting heat is transferred to the feedwater through the heatexchange tubes by both convection and radiation, and the water isthereby converted into steam which is used to operate the turbine 4.After combustion, the burnt fuel and air mixture, called flue gas, isexpelled from the fire box through the flue 10.

Not all of the heat generated by the combustion process is transferredto the water to produce steam, and often considerable heat remains inthe flue gas to be later discharged into the atmosphere from a stack 11as waste heat. Boiler flue gas temperature in the flue 10 (at point A)will range from about 1500° to 1900° F., and will fluctuate according toboiler operating demands. Downstream from the fire box, just beforefinal exit of the combustion gases from the boiler, it is normal toinclude a gas-to-water heat exchanger 12, usually called an economizer,to additionally preheat the feedwater supply using the hot flue gas witha resultant decrease in final exit flue gas temperature to between 650°and 800° F. (or higher).

In addition, prevalent design procedure is to include a regenerative airpreheater 13 to recover heat from the combustion gases and, by means ofrotating metal plates or baskets (not shown), to transfer some of thisheat to the incoming combustion air.

Although boiler thermal efficiency is increased by preheating both thewater and combustion air, final exhaust gas temperature is lowered as aresult, and this reduces the buoyancy of the exiting plume. A lessbuoyant plume will rise less high from the stack exit into theatmosphere, resulting in less mixing and dilution with the atmosphere,and it will fall more quickly to ground level in the vicinity of thelocal surroundings, increasing local measurements of pollution levels.The temperature range of exhaust gas entering the stack (at point B) isstandardized in the thermal power industry between a low of 250° F. anda high of 400° F. for discharge to either the induced draft fan 9 inletor, in most cases, directly to the stack 11 (when only a forced draftfan is used).

For sulfur bearing fuels, approximately 1 to 2 percent of the sulfurpresent in the flue gas normally converts to sulfur trioxide. The aciddew point (point of condensation) of sulfur trioxide as sulfuric acid isabout 220° F. As the regenerative air preheater plates pass from thecooler supply air side to the hot combustion flue gas side, average coldend metal temperatures (CEMT) below the acid dew point are presented tothe sulfur trioxide and this way cause condensation, deposition, andcorrosion. Some boilers use steam heat on the entering cold air to raisethe CEMT to prevent sulfur trioxide condensation. Whether or not steamheat is used, the condensation of sulfur trioxide can be avoided orminimized by maintaining flue gas temperature and the CEMT well abovethe acid dew point. Additionally, some fuels such as natural gas have ahigh water content which carries over as water vapor in the flue gas. Inthis case, the exit gas temperature is also held in the designated rangebecause of the high water vapor content which would create a densenonbuoyant and opaque water vapor plume if exhausted at temperaturesnear the condensation point of water. The latter plume formation is alsoaffected by ambient air mixing temperature.

Thus, corrosion problems and pollution problems have dictated highexhaust temperature and have prevented recoupment of the heat in theexhaust gas. By using the heat content remaining in the flue gas tofurther preheat combustion air a large gain in thermal efficiency can berealized. Heat recovery from the flue gas to the incoming combustion air(utilizing heat exchangers) represented by a drop in flue gastemperature of 40° F. results in an increase of approximately onepercent in boiler efficiency. If there were no problems with corrosionor a water vapor plume, a minimum exit flue gas temperature of 160° to180° F. would suffice to ensure buoyancy of the plume for acceptableground level concentrations even if no pollutants were removed, and theboiler efficiency would be increased significantly.

SUMMARY OF THE INVENTION

The pollution control system described herein uses ozone to producesubstantial gas phase oxidation of selected pollutants contained in theflue gas which result from combustion in the boiler, namely oxides ofnitrogen, sulfur and carbon. Before oxidation, the flue gas is cooled toa temperature range allowing optimal oxidation. Upon hydration with anycarryover contained in the boiler combustion gas and water injected intothe exhaust stream and neutralization by an alkaline solution injectedinto the exhaust stream, these aerosol pollutants are filtered throughdolomite-coated metal mesh bag filters and mechanically removed alongwith particulates normally removed by the bag filters. When the systemis used to clean the exhaust of a thermal power plant, the coolingrequired by the system can be accomplished by heat exchange withcombustion air, feedwater or fuel, thereby enhancing the efficiency ofthe power plant. Because the process removes sulfurous acid and moisturebefore entry into the exhaust stack, the exhaust gases may be cooledwell below the conventional temperature range, thereby allowing furtherenhancement of the power plant efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical arrangement of exhaust systems used in thermalpower plants.

FIG. 2 shows a first embodiment of the pollution control system.

FIG. 3 shows another embodiment of the pollution control system.

FIG. 4 shows a bag filter house with removable cartridges.

FIG. 5 shows a horizontal cross-section of the bag filter house.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 2 illustrates steam boiler exhaust system representing embodimentsof the inventions described below for use in power plants and othercombustion processes. In general, the system includes a cooler forcooling the flue gas to a temperature range in which oxidation can occurreadily, an oxidant injection system for injecting an oxidant into theflue gas stream at a point downstream of the cooler, and a highefficiency filter for removing the products of reaction. A waterinjection system for injecting water into the flue gas stream downstreamof the cooler may be included to supplement the natural water content ofthe flue gas if necessary to facilitate the removal. An alkalineinjection system may be included to neutralize the acid aerosols whichare expected products of the reaction and thus reduce the acidity of theflue gas stream entering the high efficiency filter. Additionally, acooler may be placed downstream of the high efficiency filter to furthercool the flue gas before it is exhausted from the system. Where thesystem is used to clean the flue gas of a thermal power plant, thecoolers may be cooled with combustion air, fuel, or feedwater to enhancethe thermal efficiency of the power plant.

In this first embodiment, the flue gas or combustion gas is cooled by acooler placed just downstream of the economizer or boiler flue gasoutlet. FIG. 2 shows a cooler comprised of a multi-bank, liquid coupled,regenerative heat exchanger 15 which allows precise temperature controlof flue gas temperature. The number of banks used for cooling can bevaried, as flue gas or fire box temperature varies, in order to obtain atemperature of 200° to 220° F. at Point D. Flue gas temperature willvary with power load, so that it can be appreciated that multi-bank heatexchangers will facilitate precise temperature control. Temperaturesensors 16 at Points C and D can be used to provide temperature input toa control system (not shown) which controls valves supplying a coolingfluid to the regenerative heat exchanger banks in order to maintain theproper temperature at Point D. Other systems for controlling thetemperature at Point D may be employed in alternate embodiments,including the regenerative air preheaters currently in use. While liquidcoupled regenerative heat exchangers which use flue gas to heat incomingcombustion air are depicted in the drawing, the heat exchanger couldalso be used to heat incoming feedwater or incoming fuel, and the heatexchanger may be of any suitable design. All heat exchangers in the fluegas stream may be provided with soot blowers to prevent particulatebuild-up which will interfere with heat exchange. The heat exchangerexchanges heat between the flue gas and the feedwater, fuel or, asdepicted in FIG. 2, the combustion air.

The heat exchanger is operated to cool the flue gas to a temperaturerange just above the acid dew point, but high enough to preventcondensation on the heat exchanger tubes. Induced draft fan 9 isoperated to maintain several inches of water negative pressure. In thistemperature and pressure range, the sulfur compounds, nitrogen compoundsand carbon compounds may be readily oxidized by reaction with ozone andthis temperature range is an optional temperature range for oxidation ofthese compounds. Optimization of the temperature, pressure, and flow andquantity of ozone injected allows for the controlled generation ofsulfur oxides, nitrogen oxides and carbon oxides. Acid aerosols ofnitric acid, nitrous acid, sulfuric acid, sulfurous acid and carbonicacid and mixed aerosol of these acidic products are expected to beproduced upon the condensation of the water vapor in the combustion gas.

Ozone is used as the oxidant in the first embodiment. Point D representsan injection chamber housing nozzles 14a for ozone injection to causeoxidation of nitrogen and sulfur, oxides of nitrogen, sulfur, nitrogen-and sulfur-containing compounds, and carbon that are present in thecombustion gases. In the temperature range of 200°-250° F. provided bythe heat exchangers, and a slight negative pressure of up to ten inchesof water vacuum, as provided by induced draft fan 9, oxidation ofnitrogen, sulfur, nitrogen oxides and sulfur oxides and nitrogen- andsulfur-containing compounds, with ozone occurs readily.

In this temperature range, the removal of the oxidation products is alsofacilitated by the presence of water vapor. Water vapor will always bepresent in the flue gas, but injection of a water mist may be needed tofurther increase the humidity of the flue gas to facilitate the desiredneutralization reactions to occur downstream. Accordingly, waterinjection nozzles (14b) are located at Point E to provide for injectionof high pressure atomized water which may be used to increase thehumidity and supplement the water content of the flue gas. Water willcombine with sulfur dioxide to form sulfurous acid aerosol, with sulfurtrioxide to form sulfuric acid aerosol and with nitrogen dioxide to formnitric acid and nitrous acid aerosols, sulfur dioxide, nitric oxide,nitrogen dioxide and other oxides of nitrogen at the temperatureresulting from operation of economizer 12 and heat exchanger 15. Thus,the method induces oxides of sulfur and nitrogen compounds into acidaerosol states for subsequent removal by filtration.

Point F also indicates a point where an alkaline or neutralizingsolution may be injected into the combustion gas stream which nowcontains acid aerosols produced as described above. Neutralization helpspreserve the dolomite film used in the bag filters. Neutralization alsoallows for easier handling of the acid aerosol and moisture in theexhaust stream which is removed by the bag filterhouse described below.Neutralization also allows use of less acid resistant material in thebag filters and filterhouse. The alkaline solution will be injectedthrough nozzles 14c at locations before the bag filters and combine withand be partially diluted by the condensing water vapor contained in theflue gas. The resulting solution will maintain a high, neutral orslightly acidic pH in the aqueous contacting phase flowing down and overthe dolomite films on the filter bags, thereby minimizing thedissolution of the dolomite.

For the first embodiment, dry lime, CaO, may be used, or a caustic NaOHor KOH solution may be used as the neutralizing solution.

Solid dry lime may be added directly to the flue gas, using the watervapor in the flue gas to hydrate the lime and convert it to calciumhydroxide. The lime may also be injected as a solution of water and limethrough nozzles 14d, or dispersed as powder in the inlet plenum of thefilterhouse 20. The resultant hydrated lime solution flows down over thefilter bags and provides the contacting phase for the neutralization andremoval of the aerosol species. The cascading effect of the hydratedlime solution acts as a curtain through which the flue gas mustpenetrate in order to pass through the bag and into the induced draftfan and exhaust stack. The efficient contacting of gases through thealkaline liquid phase neutralizes the sulfuric, sulfurous, nitrous,nitric and combined nitrogen-sulfur acidic compounds. The removal ofsome CO₂ and carbonic acid in this fashion with hydrated lime willresult in the precipitation of CaCO₃ solid. This cake or scale mayadhere to the dolomite film on the filter bag, but significantquantities of the precipitate are carried downward in the flowingcurtain of alkaline solution. CaCO₃ cake which accumulates on thedolomite film and bag filters will reinforce the integrity of bagfilters.

In another embodiment, the alkaline solution injected through nozzlessuch as nozzles 14d at Point F is formed with caustic sodium hydroxideor potassium hydroxide. It should be appreciated that any base or basicsolution will be useful in the process.

Thus, the exhaust stream downstream of Point F will be comprised of astream of atomized acid aerosols and neutralizing solution and anyprecipitating solids formed upon neutralization is passed to a bagfilterhouse 20. The exhaust stream may be partly neutralized at point Fand may be partly neutralized through further mixing in the bagfilterhouse and through contact with the dolomite film on the filter.

Referring to FIG. 4, a high efficiency filtration system is provided toremove the acid aerosols, alkaline solution, moisture and anyparticulate for the flue gas. The high efficiency filter of the firstembodiment comprises a revolving cartridge bag filterhouse 20 whichremoves the acid aerosols, alkaline solution and most of theparticulates. The bag filterhouse 20 uses metal mesh filter bagsintended for top flue gas entry at low temperature and "wet" conditions.The bags are pre-coated with dolomite or other filter aid material suchas diatomaceous earth. The bag filters revolve through the filterhouseto expose some bags to the flue gas stream while others are isolatedfrom the stream. In this manner, fresh filtration surface may becontinuously introduced, minimizing fluctuations in overall filterpressure drop. Filter bags are introduced and taken out of serviceexternally, where they are cleaned, re-coated with dolomite or otherfilter aid material, and tested. Exhaust gases will exit from the bagfilterhouse as clean, dry gas, the acid aerosols, particulate contentand moisture having been removed by the bag filters. The bulk of acidaerosols, neutralizing solution, excess moisture and particulates exitthe bag filterhouse in the removable cartridges and partially as anaqueous solution or slurry from the base of the filterhouse structure.The various components can be reclaimed or the ash-slurry may bediscarded in a safe manner using conventional waste disposal techniques.

Another multi-bank, liquid coupled, regenerative heat exchanger 18 isincluded which operates on cleaned, acid- and particle-free, lowmoisture gas and reduces exit flue gas temperature to the lowest levelpossible while maintaining proper buoyancy characteristics of the plumedischarge. This temperature is limited only by remaining moisturecontent of the flue gas and ambient air temperatures after it exits thefilterhouse, which will affect the temperature required to maintain aninvisible plume. As shown in FIGS. 2 and 3, heat exchanger 18 usesincoming combustion air to cool the flue gas, but the heat exchanger mayalso be aligned to use incoming fuel or feedwater.

An induced draft fan 9 may be provided to maintain a negative pressurefrom Point F to the exit of the filterhouse, preventing leakage ofunfiltered flue gas from the filterhouse. The need for the induced draftfan will depend upon the integrity of the injection chambers and bagfilterhouse, and may not be necessary if the integrity of the injectionchambers bag filterhouse can be designed so as to minimize exhaust gasleakage at operating pressures. The induced draft fan at the outlet ofthe bag filter house will also help maintain stable flow despite theincreased pressure drops that will result by operation of the variouscomponents of the system.

In an alternative embodiment, suitable for power plants using abnormallyhigh sulfur-bearing fuels, a second optimal temperature range foroxidation of sulfur oxides may be provided and a second ozone injectioncan be provided to help remove sulfur. In this embodiment as shown inFIG. 3, another independent, multi-bank, liquid coupled, regenerativeheat exchanger 19 may be located at the boiler combustion gas exit (withor without a combustion air preheater or an economizer) and anotherinjection chamber may be placed downstream of this heat exchanger.Utilizing the appropriate number of banks of heat exchanger 19 which arein operation will provide variable control of flue gas temperature from400° to 600° F. or so at point C, and the excess heat will betransferred to the inlet combustion air. The pressure at this point istypically about 3 to 12 inches of water over pressure within theoperational pressure ranges. This temperature range and pressure isoptimal for conversion of sulfur dioxide to sulfur trioxide in gaseousform by reaction with ozone. Ozone may then be injected through nozzles14e at Point C from three to ten times stoichiometric for any load levelof steam delivered. Complete conversion of carbon monoxide to carbondioxide and a partial conversion of nitric oxide to nitrogen dioxide isexpected in this stage. High efficiency oxidation of sulfur dioxide tosulfur trioxide is beneficial to the process but it is not necessary,since acidic aerosols produced directly from sulfur dioxide will beremoved by the process described above by acid-base neutralization ofsulfurous acid in the alkaline solution. It should be appreciated thatoxidation as described in this alternative embodiment may beaccomplished to remove a large portion of combustion by-products, sothat the pollution control system could be beneficially employed with orwithout the ozone injection at Point D. As an option, calcium hydroxidemay be injected at Point C for carbon dioxide removal. For this option,a damper-controlled bypass duct 17 may be included to maintain atemperature of approximately 1000° F. at the economizer exit upstream ofthe heat exchanger 19.

In the embodiments depicted in FIGS. 2 and 3, the heat removed by heatexchangers 15 and 18 and 19 is transferred to the incoming combustionair, which will increase overall boiler efficiency. As shown in FIGS. 2and 3, regenerative heat exchangers are coupled to the combustion airintake system. Heat exchangers 15 and 19 use the heat exchange tocontrol injection chamber temperature to serve the pollution controlsystem while preheating combustion air to enhance plant efficiency. Heatexchanger 18 takes advantage of the low by-product and water-vaporcontent of the clean exhaust gas to cool the exhaust gas whilepreheating combustion air to enhance plant efficiency. The pollutioncontrol system and regenerative heating system are thus interdependentand operated so as to enhance the functions of each vis-a-visindependent operation. Of course, where desired, as may be the case inincinerators, the pollution control system will still be usefullyemployed to purify exhaust gases without regenerative heating.

It may be desirable to produce ozone on-site, and the ozone generationequipment will require some power input and will also generate somewaste heat. Alteration of the feedwater cycle of each thermal power unitto exchange heat with the ozone generation process can replace orsupplement conventional feedwater, fuel or combustion air pre-heatingsystems. Thus, for example, steam normally used to preheat the feedwatermay instead be used for power generation in addition to the steamoriginally supplied to the turbine, thus increasing power generatingcapacity. By using this technique, the power used for on-site ozonegeneration may be ultimately reclaimed, reducing the overall powerrequirements for ozone production. Also, heat added to the flue gas byozone injection (and by the reactions with the flue gas components) canbe reclaimed by the heat exchangers.

FIG. 4 is a schematic representation of a new bag filterhouse, whichfacilitates use of the process described above. This figure illustratesonly the general construction and operation of the first embodiment.Other filtering methods and filterhouses may be used in the overallsystem while still incorporating the inventions described herein. Asshown in FIG. 4, the bag filterhouse 20 consists of a large cylindricalouter housing 21 which contains the individual filter bags 22 arrangedin cartridges 23 of multiple filter bags 22. As shown in FIG. 5, each ofthe cartridges are contained in cells, separated from each other byfull-length radial partitions 24 and from the inner periphery bysimilar, tangential partitions 25. The overall structure containingthese cells may be referred to as a turret. The turret rotates relativeto the outer housing 21 of the filterhouse 20. The outer periphery ofeach cell is open, but abuts the inner wall of the bag filterhouse outerhousing 21. Each cell is sealed from the others by a close radial fitbetween the outer edges of the radial partitions and the innercylindrical surface of the bag filterhouse 20 outer housing 21. A closefit will minimize leakage of exhaust gases around the partitions and outof the bag filterhouse.

All partitions are fixed at the top and bottom directly to largecircular plates 26 and 27, which also closely conform to the inner wallof the baghouse outer cylindrical surface housing of the baghousestructure, again to minimize leakage of exhaust gases around the bagfilters and out of the bag filterhouse. The circular plates and interiorpartitions form a rigid structure or turret which rotates as a unitabout the vertical centerline of the filterhouse by a drive mechanismcomprising, for example, a motor 28, gears 29 and 30, and drive shaft oraxle 31. The driveshaft also performs the function of a radial bearing.A similar shaft 32 also provides radial and thrust bearing at the bottomof the rotating cell structure. The upper circular plate 26 serves as abag suspension plate, and the lower circular plate 27 serves as a bottomsealing plate. The bag suspension plate, bottom sealing plate and thepartitions together form a turret which is rotated inside the bag filterhouse.

Two fixed circular plates 33 and 34 top and bottom, respectively, aredirectly attached (with no radial gap) to the cylindrical baghousefilterhouse structure 20 and form a ceiling and floor for thecylindrical bag filterhouse. The rotating turret is "sandwiched" betweenthese two fixed plates with minimum vertical top and bottom gaps tominimize leakage between cells through this region.

As shown in FIGS. 4 and 5, an opening 40 is provided in the outercylindrical surface of the filterhouse so that external access isprovided for bag cartridges for manual changeover from used to freshbags. A sufficient supply of cleaned and re-coated cartridges may bekept close at hand to the filterhouse for routine on-line replacement asthe bags continuously move into the changeover region. Bag cartridgesare releasably attached to flanges on the bag suspension plate by quickdisconnect clamps 36.

An inlet plenum 37 at the top of the filter structure is provided todistribute the flue gas to the entrance of each of the active cartridgecells and filter bags. After passing vertically into the filter bagscontained in each active cartridge and exiting through their sides andbottom, the clean flow from each cartridge proceeds into an outletplenum 38 where it is collected and directed to the inlet of the induceddraft fan 9.

The filter bags in all cartridges except those located in the changeoverpositions are selectively placed in the exhaust gas stream. Largecut-out apertures 39 are contained in the top fixed ceiling plate 33which correspond to the positions of the filter cartridges in the topplate of the moving structure. These apertures allow flow to enter thefilter bags when aligned with the cartridge positions. At the changeoverpositions 40, there are no apertures and the top fixed plate is solid,preventing flow from entering the cartridge cell positions wherecartridge replacement is performed. The apertures 39 in the fixedceiling plate 33 consist of truncated circular segments which exposeonly a portion of the bags in each cartridge to the entering flow.

The apertures 39 are arranged to allow flow to enter only a portion ofthe bags in each cartridge in a progressive manner as the cells rotatefrom one position to the next. This ensures that some of the bags are"shielded" from the flow at some point in the filtering process, so thatsome bag surface is constantly being exposed or blocked from the flow offlue gas, and fresh surface is continuously available for filtering.This is done to provide approximately a constant ratio between new andused bag surface, and hence, fairly constant pressure drop across theentire bag filterhouse during an entire cycle as fresh bags areintroduced. Similar apertures in the bottom sealing plate 27 and thefixed floor plate 34 allow the flow to enter the outlet plenum 38 fromthe exit of the cartridge cells. These apertures are all of constant,circular shape except at the changeover positions in the bottom fixedplate where there are no apertures, and the flow is blocked, similar tothe top. The effluent from the bag filters can be drained or pumped outof the filterhouse.

Upon rotation of any particular bag cartridge 23 into the changeoverpositions 40, the bag cartridge may be removed by releasing quickdisconnect clasps 36. The bag cartridge may be removed and replacedwhile the cell containing the bag cartridge remains in the changeoverposition. It may be necessary to avoid release of exhaust gases into theambient air near the changeover positions, perhaps to protect workersfrom the exhaust gas. For this purpose, the first of the two radialsections of the changeover position may be sealed and provided with aflushing system (not shown). The flushing system can be operated to drawout exhaust gases and flush the section with clean, cool ambient air.

In an alternative embodiment, the structure holding the bag cartridgesmay be held stationary and can be affixed to the outer housing while theceiling and floor plates rotate to sequentially expose new cartridges tothe flue gas stream. As shown in FIG. 5, in this embodiment the outerhousing is provided with doors 41 for each cell which may be opened whenthe upper and lower plates have revolved to seal off the cell so thatthe bag filters may be removed for replacement or reconditioning.

While the specific embodiments of the bag filter house may bebeneficially used in the practice of the invention, it is contemplatedthat other high efficiency filters may be used in the pollution controlsystem.

While the system and its components have been described according to thedetails of the first embodiments, it can readily be understood that theinventions inherent in the described embodiments can be practiced in avariety of other specific embodiments which incorporate the inventions.The claims presented below are intended to cover the inventive conceptsand are not intended to be limited to the specific embodimentsdescribed.

We claim:
 1. A system for purifying the exhaust gases of a thermal powerplant, including preheating fuel, combustion air, or boiler water,removing pollutants from combustion gases and discharging the purifiedcombustion gases into the ambient air, wherein said thermal power plantcomprises a boiler, a feedwater supply system which provides feedwaterto the boiler through a feedwater supply line, a firebox, a combustionair intake system which provides combustion air to the firebox, a fuelinjection system which provides fuel to the firebox, a combustion gasexhaust line for exhausting combustion gases including a flue downstreamin the from the firebox, a filter downstream of the flue, and an exhauststack downstream of the filter, said system comprising:a first heatexchanger in the exhaust line downstream of the flue, said first heatexchanger capable of cooling the combustion gases to a first temperaturerange in which at least one combustion byproduct is readily oxidized byan oxidant; a first oxidant injection system operably connected to theexhaust line and capable of injecting an oxidant into the combustiongases in the exhaust line; a second heat exchanger located downstream ofthe first heat exchanger, said heat exchanger being capable of coolingthe combustion gases to a second temperature range, said secondtemperature range being an optimum oxidation temperature range for atleast one combustion by-product, and a second oxidant injection systemoperably connected to the exhaust line to inject oxidants into theexhaust line downstream of the second heat exchanger; a basic solutioninjection system operably connected to the exhaust line and capable ofinjecting a basic solution into combustion gases in the exhaust line tocreate neutralization products; a filter for filtering the oxidizedcombustion by-products and neutralization products formed by theinjection of the basic solution from the combustion gases in the exhaustline, said filter being downstream of the points in the exhaust line atwhich the oxidants and basic solution are injected, a third heatexchanger downstream of the filter, said third heat exchanger beingcapable of cooling the filtered combustion gases to a temperature rangebelow the dew point of water.
 2. A system for purifying the exhaustgases of a thermal power plant, including preheating fuel, combustionair, or boiler water, removing pollutants from combustion gases anddischarging the purified combustion gases into the ambient air, whereinsaid thermal power plant comprises a boiler, a feedwater supply systemwhich provides feedwater to the boiler through a feedwater supply line,a firebox, a combustion air intake system which provides combustion airto the firebox, a fuel injection system which provides fuel to thefirebox, a combustion gas exhaust line for exhausting combustion gasesincluding a flue downstream from the firebox, a filter downstream of theflue, and an exhaust stack downstream of the filter, said systemcomprising:a first heat exchanger in the exhaust line downstream of theflue, said first heat exchanger capable of cooling the combustion gasesto a temperature range in which combustion by-products are readilyoxidized by an oxidant; an oxidant injection system operably connectedto the exhaust line and capable of injecting an oxidant into thecombustion gases in the exhaust line; a base or basic solution injectionsystem operably connected to the exhaust line and capable of injecting abase or basic solution into combustion gases in the exhaust line; afilter for filtering the oxidized combustion by-products andneutralization products formed by the injection of the base or basicsolution from the combustion gases in the exhaust line, wherein saidfilter is a bag filterhouse comprising:a cylindrical housing having acenter line and an inner wall, a circular ceiling and a circular floor,and also having a combustion gas inlet plenum above the ceiling and aclean exhaust outlet plenum below the floor; a revolving filter bagsuspension turret comprising an axle, a bag suspension plate disposed atthe top of the axle, a bottom sealing plate disposed at the bottom ofthe axle, said axle extending along the center line of the cylindricalhousing, a plurality of radial partition walls running from the bagsuspension plate to the bottom sealing plate and extending from the axleto the outer edge of the turret and forming radial sections for holdingfilter bags, the outer diameter of the turret including the bagsuspension plate and the bottom sealing plate and the radial partitionsclosely matching the inner diameter of the cylindrical housing; aplurality of bag filter cartridges, each cartridge comprised of aplurality of metal mesh bag filters coated on the inside with dolomite,each bag filter cartridge releasably suspended in suspension holesprovided in the bag suspension plate, said suspension holes aligned withvariable shape cutouts in the ceiling; said cutouts being shaped so asto seal some bag filters of each cartridge from incoming flow ofcombustion gas while the cartridge is below the cutout during itsrotation on the turret, each of said cutouts differing from the othersin shape so as to expose different bags of the cartridges as thecartridges move under each cutout; at least one change-over section inthe bag house outer cylinder defined by an opening in the outer cylinderlarge enough to allow removal of the bag filter cartridge, the portionof the ceiling above the change-over section being solid and notprovided with openings, the portion of the round floor under thechange-over section being solid and not provided with openings, suchthat the changeover section is open to the atmosphere while other bagfilter sections are in the combustion gas exhaust stream; said filterbeing downstream of the points in the exhaust line at which the oxidantand base or basic solution are injected, a second heat exchangerdownstream of the filter, said second heat exchanger being capable ofcooling the filtered combustion gases to a temperature range below thedew point of water.
 3. A system for purifying the exhaust gases of athermal power plant, including preheating fuel, combustion air, orboiler water, removing pollutants from combustion gases and dischargingthe purified combustion gases into the ambient air, wherein said thermalpower plant comprises a boiler, a feedwater supply system which providesfeedwater to the boiler through a feedwater supply line, a firebox, acombustion air intake system which provides combustion air to thefirebox, a fuel injection system which provides fuel to the firebox, acombustion gas exhaust line for exhausting combustion gases including aflue downstream the firebox, a filter downstream of the flue, and anexhaust stack downstream of the filter, said system comprising:a firstheat exchanger in the exhaust line downstream of the flue, said firstheat exchanger capable of cooling the combustion gases to a temperaturerange in which combustion by-products are readily oxidized by anoxidant; an oxidant injection system operably connected to the exhaustline and capable of injecting an oxidant into the combustion gases inthe exhaust line; a base or basic solution injection system operablyconnected to the exhaust line and capable of injecting a base or basicsolution into combustion gases in the exhaust line; a filter forfiltering the oxidized combustion by-products and neutralizationproducts formed by the injection of the base or basic solution from thecombustion gases in the exhaust line, wherein said filter is a bagfilterhouse comprising:a cylindrical housing having a center line and aninner wall, a circular ceiling and a circular floor, and also having acombustion gas inlet plenum above the ceiling and a clean exhaust outletplenum below the floor; said circular ceiling and circular floor beingrotatable within the housing, together with a motor operably connectedto the ceiling and floor to rotate the ceiling and floor; a bagsuspension turret comprising an central post, a bag suspension platedisposed at the top of the axle, a bottom sealing plate disposed at thebottom of the axle, said central post extending along the center line ofthe cylindrical housing, a plurality of radial partition walls runningfrom the bag suspension plate to the bottom sealing plate and extendingfrom the central post to the outer edge of the turret and forming radialsections for holding filter bags, the outer diameter of the turretincluding the bag suspension plate and the bottom sealing plate and theradial partitions closely matching the inner diameter of the cylindricalhousing, said bag suspension plate and bottom sealing plate being fixedalong their outer diameter to inner wall of the cylindrical housing; aplurality of bag filter cartridges, each cartridge comprised of aplurality of metal mesh bag filters coated on the inside with dolomite,each bag filter cartridge releasably suspended in suspension holesprovided in the bag suspension plate; a circular ceiling and circularfloor being rotatable within the housing, together with a motor operablyconnected to the ceiling and floor to rotate the ceiling and floor; saidcircular ceiling characterized by radial segments corresponding to theradial segments of the turret, at least one of said segments beingcomplete and without any cutout, the remaining said segments beingprovided with variable shape cutouts positioned above the bag cartridgessuch that upon rotation of the ceiling plate the cutouts move over thebag cartridges and variably expose bags to the combustion gas stream,said floor also characterized by radial segments corresponding to theradial segments of the turret, at least one of said segments beingcomplete and without any cutout, the remaining said segments beingprovided with exhaust holes through which combustion gases can flow intothe bottom plenum; a plurality of doors in the outer housing, each ofsaid doors providing access to at least one cell of the turrets saiddoors providing access to the bag cartridges; said filter beingdownstream of the points in the exhaust line at which the oxidant andbase or basic solution are injected, a second heat exchanger downstreamof the filter, said second heat exchanger being capable of cooling thefiltered combustion gases to a temperature range below the dew point ofwater.
 4. A bag filterhouse comprising:a cylindrical housing having acenter line and an inner wall, a circular ceiling and a circular floor,and also having a combustion gas inlet plenum above the ceiling and aclean exhaust outlet plenum below the floor; said circular ceiling andcircular floor being rotatable within the housing, together with a motoroperably connected to the ceiling and floor to rotate the ceiling andfloor; a bag suspension turret comprising an central post, a bagsuspension plate disposed at the top of the axle, a bottom sealing platedisposed at the bottom of the axle, said central post extending alongthe center line of the cylindrical housing, a plurality of radialpartition walls running from the bag suspension plate to the bottomsealing plate and extending from the central post to the outer edge ofthe turret and forming radial sections for holding filter bags, theouter diameter of the turret including the bag suspension plate and thebottom sealing plate and the radial partitions closely matching theinner diameter of the cylindrical housing, said bag suspension plate andbottom sealing plate being fixed along their outer diameter to innerwall of the cylindrical housing; a plurality of bag filter cartridges,each cartridge comprised of a plurality of metal mesh bag filters coatedon the inside with dolomite, each bag filter cartridge releasablysuspended in suspension holes provided in the bag suspension plate; acircular ceiling and circular floor being rotatable within the housing,together with a motor operably connected to the ceiling and floor torotate the ceiling and floor; said circular ceiling characterized byradial segments corresponding to the radial segments of the turret, atleast one of said segments being complete and without any cutout, theremaining said segments being provided with variable shape cutoutspositioned above the bag cartridges such that upon rotation of theceiling plate the cutouts move over the bag cartridges and variablyexpose bags to the combustion gas stream, said floor also characterizedby radial segments corresponding to the radial segments of the turret,at least one of said segments being complete and without any cutout, theremaining said segments being provided with exhaust holes through whichcombustion gases can flow into the bottom plenum; and a plurality ofdoors in the outer housing, each of said doors providing access to atleast one cell of the turret, said doors providing access to the bagcartridges.
 5. A bag filterhouse comprising:a housing having a centerline and an inner wall, a ceiling and a floor, and also having acombustion gas inlet plenum above the ceiling and a clean exhaust outletplenum below the floor; a revolving filter bag suspension turretcomprising an axle, a bag suspension plate disposed at the top of theaxle, a bottom sealing plate disposed at the bottom of the axle, saidaxle extending along the center line of the cylindrical housing, aplurality of radial partition walls running from the bag suspensionplate to the bottom sealing plate and extending from the axle to theouter edge of the turret and forming radial sections for holding filterbags, the outer diameter of the turret including the bag suspensionplate and the bottom sealing plate and the radial partitions forming aclose seal with the cylindrical housing; a plurality of bag filtercartridges, each cartridge comprised of a plurality of bag filterscoated on the inside with dolomite, each bag filter cartridge releasablysuspended in suspension holes provided in the bag suspension plate, saidsuspension holes aligned with variable shape cutouts in the ceiling;said cutouts being shaped so as to seal some bag filters of eachcartridge from incoming flow of combustion gas while the cartridge isbelow the cutout during its rotation on the turret, each of said cutoutsdiffering from the others in shape so as to expose different bags of thecartridges as the cartridges move under each cutout; at least onechange-over section in the bag house outer cylinder defined by anopening in the outer cylinder large enough to allow removal of the bagfilter cartridge, the portion of the ceiling above the change-oversection being solid and not provided with openings, the portion of theround floor under the change-over section being solid and not providedwith openings, such that the changeover section is open to theatmosphere while other bag filter sections are in the combustion gasexhaust stream.
 6. A bag filterhouse comprising:a cylindrical housinghaving a center line and an inner wall, a circular ceiling and acircular floor, and also having a combustion gas inlet plenum above theceiling and a clean exhaust outlet plenum below the floor; a revolvingfilter bag suspension turret comprising an axle, a bag suspension platedisposed at the top of the axle, a bottom sealing plate disposed at thebottom of the axle, said axle extending along the center line of thecylindrical housing, a plurality of radial partition walls running fromthe bag suspension plate to the bottom sealing plate and extending fromthe axle to the outer edge of the turret and forming radial sections forholding filter bags, the outer diameter of the turret including the bagsuspension plate and the bottom sealing plate and the radial partitionsclosely matching the inner diameter of the cylindrical housing; aplurality of bag filter cartridges, each cartridge comprised of aplurality of metal mesh bag filters coated on the inside with dolomite,each bag filter cartridge releasably suspended in suspension holesprovided in the bag suspension plate, said suspension holes aligned withvariable shape cutouts in the ceiling; said cutouts being shaped so asto seal some bag filters of each cartridge from incoming flow ofcombustion gas while the cartridge is below the cutout during itsrotation on the turret, each of said cutouts differing from the othersin shape so as to expose different bags of the cartridges as thecartridges move under each cutout; at least one change-over section inthe bag house outer cylinder defined by an opening in the outer cylinderlarge enough to allow removal of the bag filter cartridge, the portionof the ceiling above the change-over section being solid and notprovided with openings, the portion of the round floor under thechange-over section being solid and not provided with openings, suchthat the changeover section is open to the atmosphere while other bagfilter sections are in the combustion gas exhaust stream.